Spread spectrum systems operate by spreading the spectrum of the communications signal well beyond the bandwidth of the unspread signal. Motivation for using spread spectrum signals is based on the following facts: (1) These systems have the ability to reject unintentional jamming by interfering signals so that information may be communicated. (2) Spread spectrum signals minimize interference with competing users since the power transmitted is spread over a large frequency bandwidth. (3) Since these signals cannot be readily demodulated without knowing the code and its precise timing, message privacy is attained. (4) The wide bandwidth of spread spectrum signals provides tolerance to multi-path propagation. (5) Multiple access or the ability to send many independent signals over the same frequency band is possible using spread spectrum techniques. Systems employing spread spectrum methods to communicate in a secure and non-interfering manner are well-known in the art.
Spread spectrum systems may spread the communications signal using direct sequence or frequency hopping methods. In a direct sequence spread spectrum system, data bits are modulated with a spreading sequence before transmission. Each bit of information is modulated with a series of chips from the spreading sequence. The number of chips per bit defines the processing gain. A greater number of chips per bit creates a greater immunity to noise and other interference.
The band spread is accomplished by means of a code that is independent of the data, and a synchronized reception with a code at the receiver is used for despreading the incoming signal. In operation, a data signal is multiplied by a pseudo-random noise (PN) sequence having a faster data rate than the data signal to be transmitted.
Prior to application of the spreading code to the information bit stream, the information bits may undergo a series of digital operations which further increase the performance of the system. For example, the information bits may undergo differential encoding in order to be more tolerant to an incorrect phase lock in the receiver's phase locked loop (PLL). The information bits may be scrambled using a long scrambling sequence in order to further decrease the vulnerability of the system to interception.
Direct sequence spread spectrum coding may use binary phase shift keying (BPSK). In a phase shift keyed system, information is carried in the phase of the signal. Two different phases are used to denote two different digital values. Whenever the sequence transitions from a “1” to a “0” or from a “0” to a “1”, the phase of the signal transitions. Such a system is referred to as a BPSK system.
Another form of spread spectrum is called frequency-hopped spread spectrum, where the carrier frequency signal is moved (hopped) around in the band in a pseudo-random fashion. The result is an increase in effective bandwidth over time. Specifically, in a frequency hopped spread spectrum system the carrier frequency is shifted in discrete increments in a pattern generated by a code sequence. In such a system, the signal frequency remains constant for a specified time duration, referred to as a hopping time. The system may be either a fast hop system or a slow hop system. In a fast hop system, the frequency hopping occurs at the rate that is greater than the message bit rate. In a slow hop system, the hop rate is less than the message bit rate. There is also an intermediate situation in which the hop rate and message bit rate are of the same order of magnitude.
Hybrid spread spectrum systems, which use both direct sequence and frequency hopping to spread the original spectrum, come in different forms and their operation is extensively described in the literature. Spread spectrum systems that use multiple carriers are also known in the literature. What does not appear in any of the literature, however, is that the output signal of hybrid multi-carrier spread spectrum systems may have a constant envelope.
Since a conventional hybrid spread spectrum system does not transmit a signal having a constant envelope, it has shortcomings. A main shortcoming is low power efficiency, since a high power amplifier cannot operate efficiently when the input signal does not have a constant envelope. Another downside of such a system is its low bandwidth efficiency, since the common way to increase throughput is to increase bandwidth.
The present invention addresses systems and methods for generating hybrid spread spectrum signals, using both direct sequence and frequency hopping, having constant envelopes.